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Types of Telescopes

Types of telescopes

There are many different types of telescopes available for those interested in exploring the night sky. Telescopes come in a variety of designs, some of which have been around since the 1600s. 

The first type of telescope ever designed was a refractor, designed by an eyeglass maker in the Netherlands in 1608. Soon after, Galileo improved on this refracting telescope design and applied its uses to astronomy.

The three main types of telescopes are refracting telescopes, reflecting telescopes, and catadioptric telescopes. There are many variations and hybrid designs for each type. 

I consider a Dobsonian Reflector to be the best telescope type for a beginner due to its combination of ease-of-use, function, and affordability. For astrophotography purposes, a compact apochromatic refractor is best. 

Setting up my Schmidt-Cassegrain telescope for a night of astrophotography. 

If you are a beginner and looking to purchase your first telescope, it is recommended you have a clear vision for what you want to use it for (i.e. visual, photography, planetary, deep-sky, etc.), your available budget, and the level of complexity you want to take on for your equipment set-up.

The main goal of any astronomical telescope is to gather light. People often focus on the magnification of the telescope, which is actually less important than the instrument’s ability to collect more light than the observer’s unaided eye can.

When choosing a telescope there are many different characteristics that come into play, but arguably the two most important factors will be your desired focal length and aperture.

astrophotography telescope

A 150mm diameter refractor telescope. 1050mm focal length at F/7.

Key Telescope Specifications:

  • Focal Length (Magnification)
  • Focal Ratio (Light Gathering Ability
  • Aperture (Size of Objective Lens/Primary Mirror)

The focal length determines your field of view and will impact how far you will be able to see through your telescope and aperture will determine how much light you will capture and the level of detail.

These two factors will create a unique viewing experience based on the type of experience you are looking for. Below are the three main types of telescopes, including the advantages, disadvantages, and a few examples of each type of telescope that I have experience with.

As an astrophotographer first, visual astronomer second, I understand the confusion that some folks have about choosing their first telescope. Unfortunately, there is no one-size-fits-all telescope for great visual observations, and practical astrophotography too. 

In the video below, I discuss the type of telescope I would recommend for a complete beginner (a manual Dobsonian reflector).

Refractor Telescopes

A refractor telescope has a glass objective lens as the main focusing unit to collect light at the front of a long tube. This lens is usually made up of two or more lens elements where light is bent (or refracted), as it passes through the tube, to produce a clear image and reduce distortion.

In the past, I have written about my thoughts on using a refractor for astrophotography, and that I think it is the best choice to get started in deep-sky astrophotography. 

Modern refractor/refracting telescopes come in two styles: achromatic and apochromatic. Each of these styles is designed to reduce chromatic aberration (i.e. color fringing or color distortion/dispersion) differently, which is a common problem in lenses (i.e. refractors) when colors are refracted/bent incorrectly.

Radian Raptor 61 telescope

This happens because the lens is either unable to bring all wavelengths of color to the same focal plane and/or when they are focused at different areas on the focal plane and causes a mismatch in color at the focal point.

There are ways to reduce chromatic aberration. One method is to use multiple compensating lenses to counteract chromatic aberration. Another method is to uses a long objective focal length (distance between the focus and the objective) to minimize the effect.

Apochromatic refractors (apochromat) have objectives built with special, extra-low dispersion materials. They are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane. The residual color error (tertiary spectrum) can be down to an order of magnitude less than that of an achromatic lens.

apochromatic lens design

Such telescopes contain elements of fluorite or special, extra-low dispersion (ED) glass in the objective and produce a very crisp image that is virtually free of chromatic aberration. Due to the special materials needed in the fabrication, apochromatic refractors are usually more expensive than telescopes of other types with a comparable aperture.

Advantages to Refractor Telescopes

Although you have to deal with chromatic aberration, refractor telescopes can be more reliable. After the initial alignment, their optical system is more resistant to misalignment than that of a reflector telescope.

The glass surface inside the tube is also sealed from the atmosphere protecting it from dust and other lens disturbances, reducing the need to clean the lens.

High-quality refractors can also provide crisp, high-contrast images with high magnification, making refractors a great telescope for astrophotography and planetary/lunar observing.

They are usually much smaller and portable than other types of telescopes too. This means that you will not require a large equatorial telescope mount as you would for some of the larger telescope types. 

Sky-Watcher Esprit 100

The Sky-Watcher Esprit 100 ED APO.

Refracting telescopes I have used:

A refractor telescope is my top choice when it comes to your first astrophotography telescope. The compact size, crisp optics, and wide-field imaging possibilities are perfect for newcomers. 

Reflector Telescopes

Unlike a refractor, a reflector telescope uses mirrors (that bend inward) to reflect light that has traveled down a tube, into a secondary mirror near the top of the tube directing light into the eyepiece.

A reflector telescope is said to be the best value for your money, as they offer the most aperture for your dollar. For example, a 6-inch diameter refractor telescope can cost up to 10X as much as a 6-inch Newtonian reflector. 

Many reflector/reflecting telescopes are great for viewing planets (such as Mars, Jupiter, or Saturn) and the simplistic design makes them easy to build and also means they are often very affordable.

The most common form of this telescope is the Newtonian reflector, which was (you guessed it) invented by Isaac Newton

A Newtonian reflector includes a curved, dish-shaped primary mirror to collect light at the bottom of the telescope. At the top of the telescope, a small diagonal secondary mirror directs the light from the primary mirror to the eyepiece, found on the side of the telescope.

The best telescope for beginners

Looking through a Newtonian Reflector telescope on a Dobsonian mount. 

The use of inward-bent mirrors in place of a glass lens also eliminates chromatic aberration as wavelengths reflect off the mirror to a single point.

The objective mirror on a reflector is supported along the back end of the telescope so the mirrors can be made very large. Compared to a refractor of the same size, a reflector telescope is cheaper to make and can therefore be cheaper to purchase.

I consider a Dobsonian mount design to be the best telescope for beginners. It offers a large aperture with plenty of light-gathering ability, a practical format for viewing, and an affordable price tag.

Newtonian reflectors work well at focal ratios from f/4 to f/8, and unlike a refractor telescope, can provide a wide field of view relative to its aperture. 

A few disadvantages with this type of telescope are that they can easily fall out of alignment (collimation), and with an exposed/wide-open tube, the optics need frequent cleaning.

With a secondary mirror used to redirect the light into a more convenient viewing spot, this mirror can produce diffraction effects.

Reflecting telescopes I have used:

Orion Newtonian Reflector Telescope

My 8″ Orion Newtonian Reflector Telescope.

What is Collimation?

In general, reflector telescopes require more upkeep and maintenance than other telescope designs. Reflectors must be regularly collimated to perform at their optimal ability. 

The process of collimating a reflector telescope involves precisely aligning the mirrors in the telescope using specialized, simple tools. A laser collimator is a handy tool to consider purchasing if you own a reflector telescope. 

The diagram below displays what you see through a collimating eyepiece down a reflector’s focuser tube.

collimation

How to Collimate Your Newtonian Reflector Telescope (Sky and Telescope).

Ritchey-Chrétien Telescopes

Ritchey Chretién telescopes (RC’s) use a two-mirror design to bring an image into focus. This mirror system places the RC telescope in the reflector category.

RC’s are fantastic telescopes for long focal length astrophotography of distant, small objects in the night sky. In fact, the Hubble Space Telescope is a Ritchey Chretién.

Knife edge light baffles

The iOptron Photron RC6 (Ritchey Chretién).

The secondary mirror in a Ritchey Chretién is held up by four struts (spider vanes), just like a Newtonian telescope. This creates diffraction spikes in the bright stars of an astrophoto, which most agree adds to the appeal of the image. 

The Ritchey-Chrétien design is designed to offer a coma and chromatic aberration-free experience thanks to their well-corrected optics. 

In late 2018, I had an opportunity to test my first Ritchey Chrétien telescope (iOptron Photron RC6). By design, this catadioptric telescope uses hyperbolic primary and secondary mirrors.

Ritchey Chretien Telescope Design

 

What is an RC Telescope? Discussion on Quora.

Catadioptric Telescopes

A catadioptric (or compound) telescope, uses both lenses and mirrors. The greatest appeal of this type of telescope is that they are very compact with their tubes two to three times as long as wide.

The result is that you can obtain a large-aperture, long-focus telescope that’s very transportable. This type of telescope does need occasional optical collimation, but this is not something I have ever experienced myself. 

The image below shows an 11-inch Schmidt-Cassegrain telescope with a native focal length of 2800mm at F/10. Thanks to its large aperture and long focal length (high magnification), this is the telescope I use to photograph planets in impressive detail. 

Celestron Edge HD 11

My Celestron Edge HD 11 Schmidt-Cassegrain Telescope. 

Similar to refractors, the tubes for these telescopes are seal to keep out dirt and dust, which is a big plus especially if you are in an environment prone to these types of conditions.

To handle dew/moisture, a lens hood (dew shield) can help to prevent moisture from accumulating on the exposed corrector plate.

A Schmidt-Cassegrain (such as a Celestron C8, C11, etc.) is a catadioptric telescope that combines a Cassegrain reflector’s optical path with a Schmidt corrector plate to make a compact astronomical instrument that uses simple spherical surfaces.

Catadioptric telescopes I have used:

Another catadioptric telescope that is widely used for astronomy is the Maksutov-Cassegrain or “Mak” for short. Unlike refractors and reflectors, this telescope design does not suffer from coma or chromatic aberration. 

These telescopes are great for lunar and planetary viewing, as well as terrestrial daytime use. The slower focal ratios of this type of telescope mean less light is reaching the eyepiece (or your camera) than some of the fast focal ratio telescopes out there. 

A common “Mak-Cass” telescope on the market today is the Celestron NexStar 4SE. This is a 4″ (102mm) Maksutov-Cassegrain design in a compact, computerized (GoTo) package.

Celestron NexStar 4SE

The Celestron NexStar 4SE Maksutov-Cassegrain Telescope. 

Astrophotography

If deep-sky astrophotography is your goal, choosing the right telescope will come down to the types of objects you wish to shoot. 

For large nebulae and galaxies, a high-quality triplet apochromatic refractor is a great choice. For small galaxies and planetary astrophotography, a long focal length SCT is a better fit.

Almost any telescope is suitable for photographing the moon because it is so big and bright. Other objects in space, such as comets, star clusters, and galaxies can be more challenging.

To capture images of space through a telescope, you must attach a camera to the telescope using the appropriate adapters.

You can also handhold your camera up the eyepiece of your telescope to take a picture of the moon or bright planets, but this method can be challenging and yield poor results.

Sky-Watcher OSC Combo

A typical astrophotography camera and telescope setup.

The majority of the deep-sky astrophotography I have taken were using apochromatic refractor telescopes. The camera was threaded directly onto the focuser drawtube of the optical tube assembly (prime focus), utilizing the native focal length of the instrument.

This means that to photograph a variety of objects in space (big and small), an assortment of different telescope types must be used. 

best astrophotography images

Astrophotography images captured in my backyard through various telescopes.

The Bottom Line

The best type of telescope for you will depend on your intended uses. If you are a casual observer of the night sky and want a telescope that provides a painless, enjoyable experience, an 8″ Dobsonian reflector is probably your best bet

If you’re an amateur astrophotographer like me, a compact, wide-field apochromatic refractor will likely be your most valued telescope. The design of your telescope will determine its size, magnification, focal length, and light-gathering ability. 

Because these aspects change based on design, there is no “one-size-fits-all” telescope for every scenario.

With that being said, many seasoned amateur astronomers will recommend having a dedicated telescope for visual astronomy (one that is easy to transport and offers comfortable views), and one that is strictly used for astrophotography purposes. 

About Me:

Trevor and Ashley

Trevor Jones and his wife Ashley.

I have been observing and photographing the night sky using various telescopes for over 10 years. The largest telescope I have ever observed an object in space through was a 36″ Dobsonian reflector at the Cherry Springs Star Party in 2018!

Helpful Resources:

types of telescopes